You can just cut a round hole in the middle of the round end of a carto condom.
Or I carry a brand new drip tip with me in it little plastic zip lock - I
vape with a drip tip as a mouthpiece instead of the end cap anyway so it would be easy for me to pull my drip tip off and stick a fresh one on. I get mine at LiteCigUSA.net and I know the round black delrin and the plain clear colored ones fit my KR8 and Boge cartos. The swirl colored plastic ones are a bit too tight and I have not tried the flat tips.
I wouldn't put my mouth on something someone else has had in their mouth. I have never had cold sores (oral herpes or any herpes) and you can transmit viruses that way. I would not want to chance a life-long virus just to try something. Only exception is a jug or jar of moonshine at a fiddle festival - I pretend moonshine will kill anything.